Clickable Signs

I'm trying to come up with a way to create a compact number pad that will allow the player to enter a 3-digit number into a scoreboard objective. I've got an idea on how to make it work using a system of command blocks that add each number times its order of magnitude to the objective (i.e. to get the number 491, pressing 4 would add 400, pressing 9 would add 90, then pressing 1 would add 1). I've got a pretty good idea on how to make this work using redstone/command blocks (though honestly it might be a bit over-complicated), but I'm trying to come up with a compact way of giving the player a numberpad to enter on.

The easiest would be a 4x3 wall of buttons, but I was wondering if there was a way to take advantage of clickable signs. I know you can right click a sign to run a command using clickEvent, but is there a way to have multiple, separate click events in the same sign?I've tried using the following: